The present invention relates to the storage and transportation of acetylene.
Acetylene has utility in industry, in particular, metal fabrication such as gas welding and gas cutting operations but has the disadvantage that it is highly unstable. If an ignition source is present, pure acetylene under pressure as low as 1.4 bar absolute will decompose with violence.
One known method of stabilising acetylene is to dissolve the acetylene in a suitable solvent, for example acetone, to lower its activity. The resulting solution is then absorbed in a porous mass or filler to inhibit the decomposition. With this known method, using acetone as the solvent, acetylene gas cylinders have a limiting safety pressure of 18.7 bar absolute at 15.degree. C.
The main disadvantages of this known dissolved acetylene storage system are low storage capacity, low gas withdrawal rates, and no bulk storage or transportation capabilities.
An alternative to dissolved acetylene is to dilute the acetylene gas with another gas. Hydrocarbons, nitrogen, carbon dioxide, carbon monoxide and ammonia are the most common gases used to dilute and thereby stabilise acetylene. Dilution with 49% by volume nitrogen or 42% by volume carbon dioxide is needed to avoid acetylene decomposition at ambient temperature and a pressure of 5 bar a. Although the addition of diluents increases the pressure at which acetylene can be handled safely, the storage capacity and bulk transportation capability of acetylene are not improved.
Another alternative is to liquefy acetylene in a solvent at low temperatures, for example -90.degree. C. at atmospheric pressure. For example, in UK Patent Number 729748 there is described a process for producing dissolved acetylene in which gaseous acetylene is dissolved at atmospheric pressure at a temperature of -94.degree. C. or below in a solvent such as liquid carbon dioxide preferably in admixture with acetaldehyde and methylene chloride. The disadvantages are the high cost of the extreme cooling, the change of composition during withdrawal of either the vapour or the liquid and, the low pressure of the acetylene stored.
A third alternative is to store or transport liquid mixtures of acetylene and for example acetone or dimethylformamide at a temperature of -50.degree. C. In this case, the equilibrium pressure is higher than atmosphere and, the vapour has to be stabilised by adding a gas insoluble in the liquid like, nitrogen, noble gases or carbon monoxide. The disadvantages are the difficulties in maintaining a safe gas composition and the contamination of acetylene by the other component of the mixture.
A fourth alternative is to store or transport acetylene in carbon dioxide as described in EP Patent Publication Number 0740104 as a mixture, liquid-vapour or solid-vapour. The advantages of this system are constant compositions of the liquid and vapour phases during withdrawal of either phase, when operated as an azeotropic mixture. High acetylene content in the vapour, liquid or in the solid mixtures is produced with a wide range of pressures and temperatures at which the mixtures are stable.